Generally, this disclosure relates to thermal management of downhole tools. More particularly, this disclosure relates to thermostats for downhole tools and methods of using the same.
A Dewar flask is a passive device that is often used in a downhole tool for maintaining the temperature of sensitive components. It is very efficient at keeping the heat of the wellbore environment out of the cavity formed in it. For this reason, it is advantageously used in hot portions of wellbores. It allows the downhole tool to remain in the wellbore for longer durations before the sensitive components located in its cavity reach their maximum temperature limitation.
However, a Dewar flask also keeps the heat inside its cavity, even in cold portions of wellbores. If components located inside the cavity are generating heat, the heat may not dissipate in the cold portions of the wellbores, causing its inside temperature to progressively increase. Accordingly, in cases where the components dissipate heat, the duration during which the downhole tool may remain in the wellbore before the components located inside the cavity reach the maximum temperature limitation may be shortened. In such cases, the downhole tool is often tripped out of the wellbore, either for cooling the Dewar flask down to a suitable temperature or for replacing the Dewar flask with another one already at a suitable temperature. Then, the downhole tool is tripped back in the wellbore. This tripping of the downhole tool may waste much rig time, be a costly operation, and be potentially performed in hazardous conditions.
A conventional method to mitigate the temperature increase in a Dewar flask may be to refrigerate the Dewar flask and the components located in its cavity to a temperature lower than the ambient temperature prior to tripping the downhole tool into the wellbore. However, this method may not be suitable in cases where one or more of the components located inside the Dewar flask has a minimum operating temperature that is close to or above the ambient temperature. Gyroscopes may be an example of such components. Gyroscopes are typically used in survey tools. Many gyroscopes have a temperature range, for example between 70 deg. F. and 260 deg. F., where they operate optimally.
Another conventional method to mitigate the increase of temperature in a Dewar flask may involve selectively cooling the components located in its cavity. For example, upon detecting that one or more of the components has reached a maximum temperature limitation, a fluid may be circulated to transport heat from the cavity to the wellbore environment, or an active cooler may be turned on. However, this method may require a relatively complex and heavy circulation system or an active cooler that consumes electrical power to provide heat transfer. The added complexity and weight may not be suitable for downhole tools conveyed via slickline or wireline.
Thus, there is a continuing need in the art for thermal management of downhole tools.